In the construction of many electronic devices, it is necessary to form a large number of electrical connections between two closely spaced parallel surfaces. For example, this occur with flipped chips attached to multi-layer substrates in the construction of multi-chip modules (MCM's), in forming interlayer connections in circuit boards or in LCD display panels, in surface mounting of pad array chip carriers which have a grid array of contact pads on the bottom surface where each of the individual pads must be electrically connected to a corresponding conductive pad on the mother board or other mounting surface, and the like.
The number of contact points which must be formed, per unit area, between two opposing parallel surfaces, is called the "interconnect density." The interconnect density has been increasing rapidly in recent years. In the early 1980's, contacts within or at the base of circuit boards or other components were generally spaced on 100 mil "pitch", i.e. with center-to center spacing of 0.100" (about 2.5 mm) [1 mm=39.4 mils], yielding an interconnect density of 100 contacts per square inch. In 1993, the physical contacts have become smaller and the distance between contacts reduced to 10 mils and often less. Thus the interconnect density has increased to 10,000 or more per square inch.
The most widely used method to make the higher density closely spaced interconnects is by soldering, usually by a tin-lead alloy solder. Soldering requires that the electrical contact areas on the surfaces being joined be "wettable" by the solder when it is molten so that the cooled solder will adhere thereto. Generally this is done by causing the areas at which electrical interconnects are needed to have a surface of a wettable metal such as an oxide-free copper, gold, or other suitable metal, and to have the wettable metallic areas surrounded by non-wettable areas of, for example, an oxide or polymer to which the solder will not adhere. When solder is melted between the two opposing surfaces, surface tension pulls the molten metal into droplets which contact only the wettable metal contact areas and insures that there is no solder, and therefore no electrical contact after cooling, between adjacent solder "bumps" or balls. After cooling, the contact areas are electrically connected across the space between the two surfaces by the solder bumps. Hence, a large number of closely spaced contacts can be established. Solder bumps 4 to 5 mils in diameter on 10 mil centers can therefore satisfy the current requirements for up to 10.sup.4 individual contacts per square inch.
There are, however, many problems with the use of tin-lead solders. The lead is toxic. The fluxes and cleaning solvents used are environmental hazards. The formation of wettable and non-wettable areas on surfaces is expensive and time consuming. Tin-lead solder is relatively brittle and provides little shock resistance. If the two opposing surfaces differ in coefficient of thermal expansion, a solder-bump assembly may suffer cracking (hence electrical failure) of individual solder bumps when the assembly is cooled. While solder bumps are useful to attach silicon dies to ceramic or silicon multi-chip modules (MCM-C's or MCM-D's), the thermal expansion issue has generally precluded using solder to attach silicon dies to laminated Kapton/copper multi-chip modules (MCM-L's) or to other copper-plastic laminated circuit boards. Kapton is a Dupont polyimide film.
Attempts to avoid the use of tin-lead solder bump interconnections have included the development of anisotropic "Z-axis" polymeric adhesives. A Z-axis adhesive is intended to join two horizontal surfaces in a manner which conducts electricity vertically between the two surfaces (the Z axis) only in desired contact areas while not conducting electricity between contact points in the horizontal plane of the adhesive (the X and Y axes). Most currently used anisotropic adhesives consist of random dispersions of metal particles (often gold-plated nickel spheres) in a flexible polymeric film. Electrical conductivity in the X-Y plane is prevented by using a very low metal particle loading. When the flexible film is compressed between two opposing conductors, it is hoped that a sufficient number of individual metal particles will form the necessary electrical connections. Most Z-axis adhesives require (i) heat to cause the polymer phase to flow, wet, and cure, and (ii) pressure to force and hold the surfaces in contact. When the film is placed between two surfaces under heat and pressure, the polymer bonds to the surfaces and the particles are forced into contact, establishing conductivity in the Z-axis only. In an attempt to avoid using heat, other Z-axis systems depending only on mechanical forces to maintain contact have been suggested. Spitz, S. L., Electronic Packaging and Production, p. 12, Sep. 1989, has described a method in which spring-loaded metal clips are used to maintain pressure across a sheet of silicone rubber which is randomly loaded with conductive metal particles. Substantial pressure is needed to establish and to maintain the electrical contacts, relying on mechanical forces and not a true chemical bond between the conductive metal particles and the contact pads. Whether heat is used or not, the number of metal particles per unit area must be relatively small to prevent accidental conductivity in the X or Y direction and thus the Z-axis conductivity must also be small.
In an attempt to improve the performance of Z-axis systems and provide interfacial electrical contact at specific points, rather than relying on possibility of contact due to randomly dispersed conductive metal balls, the preparation of Z-axis connectors having conductive regions precisely located on stripes or dots separated by non-conductive regions has been suggested. Bochoff, L., "Guidelines for Designing Elastomeric Connectors," Connection Technology, 1987, and Reinke, R. R., "Interconnective Method of LCD LSI's using Anisotropic Conductive Film Connectors," Proceedings 1991 Electronic Components and Technology Conference, IEEE, 1991, p. 355, have described "zebra stripe" or "multiple dot" connectors which are generally made by forming regions of electrically conductive (carbon powder-filled) silicone rubber within regions of non-electrically conductive silicone rubber (no carbon powder). Although these silicone rubber connectors are in commercial use, they depend on pressure to hold the conductive region of a cured conductive elastomer against an opposite contact surface. In addition, the carbon-filled dots and zebra stripes inherently have relatively low electrical conductivity, a level adequate for simple LCD displays, for low cost wristwatches and calculators and the like, but inadequate for many high performance devices.
Another attempt at avoiding the limitations of the above Z-axis adhesives is disclosed in U.S. Pat. No. 4,548,862 in which conductivity between two surfaces is provided at specific (rather than random) points. This is done by filling a flexible pressure-sensitive adhesive film with metal particles having a ferromagnetic core and a gold or silver plated electrically conductive surface. The particles are then oriented and concentrated in columns at specific points by use of a magnetic field, similar to the process used to make magnetic recording tape. The method results in low conductivity per contact point due to the low conductivity of the iron oxide filler particles.
A still further attempt to produce a Z-axis system and provide interfacial electrical contact, particularly for flip chip die attach, is that of Hogerton, P. B. et al, Material Research Society Symp. Proceedings, Vol 154, pp 415-24, 1989, in which nickel or nickel-gold plated polymeric balls are used to form "pressure engaged interconnectors" by inserting the plated 0.001" (1 mil) (25.4 micron) diameter polymeric balls in selected areas of a non-conductive epoxy adhesive and then compressing those particles. When the bonding process generates a sufficiently high pressure to compress the polymer particles to about 60% of initial size, then they can provide electrical conductivity in the Z-direction. It is reported that it takes about 10 such particles per electrical connection to have conductivity. Use of this concept requires the use of a highly complicated and expensive machine to compress the adhesive while maintaining both compression and exact planarity during the cure of the epoxy adhesive around the polymeric balls. The adhesive bond which is formed is only about 0.5 mil (12.7 micron) thick--too thin a layer to accommodate differences in thermal expansion between many materials. A significant loss in conductivity is reported when the adhesive bond experiences temperatures greater than about 70.degree. C. and after long times under compression when the polymer particles "creep."
Accordingly, it is an object of the present invention to overcome the numerous disadvantages of the prior art interconnect methods and products used in an attempt to achieve high density interconnection.
It is a further object of this invention to form multiple layer circuits by relatively simple means.
It is a further object of this invention to form pad chip carriers without wire or solder bonds.
It is a further object of this invention to form solder-free laminated multi-chip modules.
These and still further objects will be apparent from the ensuing description and claims of this invention.